1. Field of the Invention
This invention relates to a circuit for converting a high level DC voltage to a high frequency voltage for use for powering electrical equipment.
2. Description of the Prior Art
Prior art power switching circuits for converting a high level DC voltage to a high frequency voltage employ two power transistors wherein the emitter of one transistor is connected to the collector of the other transistor. A high level DC voltage is connected across the two transistors. The junction of the two transistors is connected to one side of the primary of a transformer, the other side of the primary being connected to a half wave bridge across the DC power supply. The transformer has several secondaries which serve as separate channels for the high frequency voltage produced. An oscillator, comparator and base drive unit is provided which basically is an oscillator typically oscillating in the region of 20-30 khz. The output of the oscillator alternately switches the two transistors on and off to convert the DC voltage to a high frequency voltage. One of the channels is fed back to the oscillator, comparator and base drive unit for regulation purposes. A start circuit employing a transformer is coupled to the oscillator, comparator and base drive unit. Circuits of this type have disadvantages since they have only one regulated channel. Moreover, not only are the two power transistors and their associated oscillator, comparator, and base drive unit a chopper (converts the high level DC voltage to a high frequency voltage) but they also act as modulator. The net effect is that it is impossible, over the very large dynamic ranges which occur, to maintain symmetrical positive and negative areas of the waveform at the junction of the two transistors. The non-symmetrical areas also occur because of the dis-similarities of the transistors. Matching of transistors, however, will not eliminate the non-symmetrical waveform since the circuit must carry out the functions of both chopping and modulation over a wide dynamic range. The transformers used employ gapped cores rather than the highly efficient toroidal non-gapped cores. This is due to the fact that a non-symmetrical wave form results in a DC component which would quickly saturate a non-gapped toroidal core. A gapped core results in a large leakage inductance which produces switching spikes. This in turn requires power switching transistors with increased voltage capabilities which increases the cost. Moreover transformers with gapped cores require more current for operation. The prior art power switching circuits require as much as 20% of the total load current for operation. The increased current for operation also requires power transistors with high voltage capabilities. In addition the capacitors of the half wave bridge must be larger. Moreover, transformers with gapped cores result in more radiation of flux and noise which interferes with associated equipment and with the AC power supply from which the DC is obtained. The prior art power switching circuits are designed such that the power switching transistors are driven hard even under no load conditions to accomodate the worse expected conditions. This adversely affects their switching times.